The present invention relates to a new and improved electrical fuse and more particularly, to a new and improved electrical fuse including a fuse element with weak spots and heavy portions. The weak spots form necks between the heavy portions and are transvese to the length of the fuse element and parallel to each other.
If resistance in an electrical circuit is abnormally low, usually due to an accidental cause, current flow will increase considerably. If the resistance approaches zero because of a short or heavy overload within the protected circuit, the current in the circuit can range from tens to hundreds of times greater than the normal current. Substantial harm can be expected to occur quickly under these conditions in the form of thermal, magnetic and arcing damage. Therefore, time is critical in removal of supply power to shorted or heavily oveloaded circuits. Damage as a result of these conditions is generally prevented by the inclusion of fuses in the supply line of the circuit to be protected. The simplest fuse is a length of thin wire which in the event of a short circuit is heated rapidly by the high current and melts away thus interrupting the circuit.
Current limiting single element fuses provide safe and reliable protection for most electrical circuits. Such fuses operate when increased fuse element temperature caused by an overcurrent flowing through them melts the element. Since the functioning of these fuses does not depend on the operation of intricate moving mechanical parts, performance characteristics are generally quite consistent and reliable. A single element fuse usually consists of one element within an insulative tube having electrically conductive end caps. Each element is defined by a plurality of heavy portions separated by weak portions also called fusible portions or weak spots. If the electrical circuit in which a fuse is included as protection experiences a short circuit or heavy overload, high current flow quickly begins melting the weak spots. As this occurs, arcing across the melting or vaporizing weak spots commences. Once the arcs are extinguished, the circuit is cleared and the potentially damaging current flow ceases. Therefore, fast arc extinguishing speeds are desirable to protect the system components from damage due to heavy overload and short circuits.
An important feature of most single and parallel element fuses is their ability to quickly extinguish arcs between the portions of larger cross sectional area after the weak spots have melted or vaporized. Another important feature of such fuses is their ability to prevent the system open circuit voltage from restriking arcs across the open weak spots of the fuse element after the arcs have been initially extinguished. A typical fuse element includes weak spots or fusible portions which extend in a direction generally parallel with the length of the element. Arcing then occurs generally in a direction parallel to the long axis of the fuse element between its heavier portions, thereby allowing greater burn back into the heavy portions and increasing the time needed to clear the circuit. This also increases the potential for the arcs to communicate with each other.
A further important feature in fuses of this type is the ability to keep the weak spots well away from the side walls of the insulative container tube, particularly during periods of overload and clearing. It is often the case, during periods of even small overloads, that fuse elements may bow along their length due to thermal expansion. If the fuse element is not properly designed or is improperly manufactured, the weak spots near the center of the fuse element may closely aproach or touch the inside wall of the insulative container tube. This is mosr likely to occur during periods of overload which may cause substantial thermal expansion and bowing of the fuse element. As the weak spots approach or touch the walls of the container tube, heat which would have contributed to weak spot melting is drawn away by the tube walls, thereby cooling the weak spots and possibly causing substantial changes in the clearing characteristics of the fuse. Such a fuse might carry a higher than rated current for a much longer period of time before clearing than would normally be expected. In addition, as clearing begins, the products of the melted or vaporized weak spots may be deposited on the inside wall of the container tube adjacent the weak spot. Arcing may be prolonged through these deposits if they are in close enough proximity to the arc.
Therefore, it is very important that the weak spots of the element be kept well away from the inside walls of the insulative tube at all times. However, many fuse elements are designed such that their weak spots may approach or touch the walls of the container tubes, particularly during an overload because of thermal expansion and bowing. Many fuses may even be assembled such that the element and weak spots approach or touch the container tube after manufacture and before any current flows through them.
An additional important feature in fuses of this type is that they must include sufficient structural integrity to avoid bending along their width or along their length through their weak spots, particularly during manufacture and high current cycling. During manufacture, the fuse elements are subjected to numerous forces from their original blanking through assembly stages to final end can attachment and soldering. Inadvertent bends at weeak spots may either break the element or cause changes in its clearing characteristics. During high current cycling, the fuse element expands and contracts as current flow increases and decreases, causing flexing and bowing of the element. An improperly designed element can place an unusually large amount of stress on already softened weak spots during high current cycling, causing metal fatigue at the weak spots and possibly premature fuse failure.
Some examples of possible fuse element arrangements are illustrated in British Patent 1,300,136. The heavy portions or portions of larger cross sectional area are aligned on both sides of the weak spots such that beinding along an axis formed through the weak spots can readily occur. Such bending could place the weak spots in contact with or in close proximity to the inside insulative tube wall. Further, high current cycling may cause tortional stresses and fatigue at the weak spots of at least some of the disclosed elements. The disclosed elements are also very susceptible to inadvertent bending during manufacture.
A second example of possible fuse element arrangements is illustrated in U.S. Pat. No. 2,682,587, wherein at least one embodiment of an element is disclosed which includes a weak spot having a current path transverse to the long axis of the element. However, only a single weak spot is shown resulting in poor clearing of the circuit because the opened circuit voltage across a single weak spot is often great enough to permit restrike of the arc during clearing. Restrikes of the arc allow potentially damaging additional overcurrents into the protected circuit and therefore, are very undesirable.
Additional examples of typical fuse elements are illustrated in U.S. Pat. Nos. 1,788,623; 2,507,747; and 3,417,357 and German Pat. No. 1,005,669. The current paths through the weak spots connecting heavy portions of the fuse elements are generally parallel to the long axis of the elements. In those cases where the current paths are not directly parallel to the length of the fuse, the path is offset by an angle which would not substantially diminish the arc burn back into the heavier portions of the element. Therefore, these fuse elements provide little or no advantage over elements having weak spot current paths parallel to the long axis of the element.